The present invention relates to the transmission of digital data including, particularly, the transmission of digital data which represents television (TV) signals.
It is generally acknowledged that some form of digital transmission will be required for the next generation of television technology, conventionally referred to as high definition television, or HDTV. This requirement is due mostly to the fact that much more powerful video compression schemes can be implemented with digital signal processing than with analog signal processing. However, there has been some concern about getting committed to an all-digital transmission system because of the potential sensitivity of digital transmission to small variations in signal-to-noise ratio, or SNR, at the various receiving locations.
This phenomenon--sometimes referred to as the "threshold effect"--can be illustrated by considering the case of two television receivers that are respectively located at 50 and 63 miles from a television broadcast station. Since the power of the broadcast signal varies roughly as the inverse square of the distance, it is easily verified that the difference in the amount of signal power received by the television receivers is about 2 dB. Assume, now, that a digital transmission scheme is used and that transmission to the receiver that is 50 miles distant exhibits a bit-error rate of 10.sup.-6. If the 2 dB of additional signal loss for the other TV set translates into a 2 dB decrease of the SNR at the input of the receiver, then this receiver will operate with a bit-error rate of about 10.sup.-4. With these kinds of bit-error rates, the TV set that is 50 miles away would have a very good reception, whereas reception for the other TV set would probably be very poor. This kind of quick degradation in performance over short distances is generally not considered acceptable by the broadcasting industry. (By comparison, the degradation in performance for presently used analog TV transmission schemes is much more graceful.)
There is thus required a digital transmission scheme adaptable for use in television applications which overcomes this problem. Solutions used in other digital transmission environments--such as the use of a) regenerative repeaters in cable-based transmission systems or b) fall-back data rates or conditioned telephone lines in voiceband data applications--are clearly inapplicable to the free-space broadcast environment of television.
The co-pending, commonly assigned U.S. patent application of V. B. Lawrence et al entitled "Coding for Digital Transmission," Ser. No. 611,225, now U.S. Pat. No. 5,164,963, issued Nov. 17, 1992, and hereby incorporated by reference, discloses an advantageous technique for overcoming the shortcomings of standard digital transmission for over-the-air broadcasting of digital TV signals. The technique--referred to herein generically as "unequal-error-protection signaling"--comprises a particular type of source coding step followed by a particular type of channel mapping step. More specifically, the source coding step causes the television signal to be represented by two or more data streams while, in the channel mapping step, the mapping is such that the data elements of the various data streams have differing probabilities of channel-induced error, i.e., different probabilities of being erroneously detected at the receiver. Illustratively, a first one of the aforementioned data streams carries components of the overall television signal which are regarded as the most important--for example, the audio, the framing information, and the vital portions of the video information--and that data stream is mapped such that its data elements have the lowest probability of channel-induced error. A second one of the data streams carries components of the overall television signal which are regarded as less important than those of the first data stream and that data stream is mapped such that its data elements have a probability of channel-induced error that is not as low as those of the first data stream. In general, it is possible to represent the overall television signal with any number of data streams, each carrying components of varying importance and each having a respective probability of error. This approach allows a graceful degradation in reception quality at the TV set location because, as the bit error rate at the receiver begins to increase with increasing distance from the broadcast transmitter, it will be the bits that represent relatively less important portions of the TV signal information that will be the first to be affected.
Going further, the co-pending, commonly assigned U.S. patent application of L. Wei, entitled "Coded Modulation with Unequal Error Protection," now U.S. Pat. No. 5,105,442, issued Apr. 14, 1992, and hereby incorporated by reference, discloses a scheme which implements the above-described overall concept of the Lawrence et al application--i.e., provides different levels of error protection for different classes of data elements generated by the source encoding step--but which provides enhanced noise immunity via the use of coded modulation, such as trellis-coded modulation.
In the Wei arranement, more particularly, the symbols in a predetermined 2N-dimensional channel symbol constellation, N.gtoreq.1, are divided into groups, each of which is referred to as a "supersymbol." During each of a succession of symbol intervals, a predetermined number of the most important data elements are channel encoded, and the resulting channel coded data elements identify a particular one of the supersymbols. The remaining data elements, which may also be channel encoded, are used to select for transmission a particular symbol from the identified supersymbol.
The Wei approach as thus far described is similar in a general way to conventional coded modulation schemes in that the latter also divide the channel symbols into groups, typically referred to as "subsets." However, in conventional coded modulation schemes, the subsets are formed under the constraint that the minimum Euclidean distance (hereinafter referred to as the "minimum distance ") between the symbols in a subset is greater than the minimum distance between the symbols in the constellation as a whole. In Wei, however, the minimum distance between the symbols of a supersymbol is the same as the minimum distance between the symbols in the constellation as a whole. This distance property allows for greater amount of noise immunity for the most important data elements than for the other data elements, that immunity being optimized by keeping the minimum distance between supersymbols as large as possible--usually greater than the minimum distance between the symbols of the constellation. Specifically, once the supersymbols are defined, it is possible to design codes for the most important data elements based on the distances between the supersymbols, i.e., as though each supersymbol were a conventional symbol in a conventional constellation. This being so, a particular degree of noise immunity can be achieved for the most important data elements that is greater than what can be achieved for the other data elements.